1. Field of the Invention
This invention relates generally to a method for measuring current, and more particularly to a method for sensing both current and voltage levels in a current carrying cable of an electric power distribution system.
2. Description of Related Art
A variety of sensors have been developed for measuring the current in a current carrying cable, such as that of a high voltage electricity distribution system. Optical current sensors based on the Faraday effect are known in the art. Optical current sensors that use bulk glass or fiber optic cable that surround the current carrying cable have very high dynamic range but require opening the current carrying cable at installation, hence are expensive.
Optical current sensors utilizing a magnetic concentrator with bulk optics in an airgap are also known in the art. One such embodiment is discussed in an article titled ‘Use of Dual Frequency Excitation Method to Improve the Accuracy of an Optical Current Sensor,’ by Shuping Wang, et al, SPIE meeting, August, 2009. The airgap stabilizes the temperature sensitivity of the magnetic material, as discussed in the publication ‘Gapped Magnetic Core Structures,’ by Gunter B. Finke, Magnetic Metals Corporation, Camden, N.J. 08101.
However, due to saturation, the magnetic concentrator may limit the dynamic range. An electric utility has various requirements for the dynamic range of the current sensors, depending on the application. For example, for metering and demand response, dynamic ranges of about 0 to >2× the nominal current may be acceptable. When fault detection is required, a dynamic range similar to >10× the nominal current has to be measured in real time. For assessment of power quality, the measurement of the harmonics is critical, so higher bandwidths such as ˜45-˜6,000 Hz may be typically required. Smart grids deliver electricity from suppliers to consumers using digital technology to save energy, reduce cost, and increase reliability and transparency. Particularly with such smart grids, utilities and industrials will require that the same sensor be used for multiple purposes. Increasing the airgap of the magnetic concentrator may increase the saturation level but may also increase the sensitivity to adjacent fields.
Traditional sensors are typically separate for different applications. For example, current and voltage transformers are used for metering and demand response, while Rogowsky Coil or Hall effect devices are used for fault allocation and system protection. A fully fiber optic or bulk current sensor can naturally be used for all applications but is expensive and cannot be clamped to the cable.
Woods et al., U.S. Pat. No. 5,892,357, discloses an electro-optic voltage sensor for sensing voltage in an electric field, the sensor being based on Pockel's electro-optic effect. This requires a simple and contactless arrangement of the sensor with the cable.
Blake, U.S. Pat. No. 6,166,816, describes the use of one light source for a combined fiber optic current and voltage sensor. It is, however, difficult to make a clamp-on version of the current sensor disclosed. The electric utility can use it during a new set up or take apart the current carrying cable for installation.
Ishiko et al., U.S. Pat. No. 4,999,571, describes a clamp-on optical current and voltage sensor. The sensor is attached using a two part process that involves a linear slide and rotation. The voltage sensor is based on a capacitive divider that has no ground connection. The ground reference is created by the virtual capacitance between the sensor and the ground. This virtual capacitance changes with in the atmosphere (e.g., humidity, dust), mobile conductive masses such as motor vehicles, and electromagnetic interference from adjacent phases or other sources. Furthermore, the sensor relies upon a mechanical adjustment for accurate readings. The sensor must be attached to the line when the voltage to the line is off. The crystal used in the current sensor is a garnet crystal, which is temperature sensitive. The sensor also uses quarter wavelength plate for the voltage sensor, and this wave plate is also temperature sensitive.
Ykymyshyn et al., U.S. Pat. No. 7,279,870, discloses a method of measuring a current based on multiple Hall-effect based semiconductor sensors combined with electronics and compensated by a source with a reference AC voltage. This method requires installing a solid state electronics adjacent to the power cable and is therefore less reliable due to the exposure to the transients or the effects of lightning on the cable.
Bjorn, U.S. Pat. No. 7,068,025, teaches a simplified sensor based on the Faraday effect that relates a rotation of the plane of polarization in proportion to the intensity of the component of the magnetic field in the direction of the beam of light. Ampere's law relates the integrated magnetic field around a closed loop of a conductor to the electric current passing through the loop. The Bjorn patent teaches a method that samples only one point around the conductor.
This method is sensitive to the magnetic field of an adjacent phase or to the magnetic interferences with other sources. To compensate for those errors, software corrections are utilized, by comparing the readout to a reference current sensor that surrounds the conductor. This compensation method is not accurate when there are changes in the installation. Even common factors such as wind the passage of nearby cars can change the configuration of the measured magnetic field.
C. V Temple et al., U.S. Pat. No. 2,709,800, teaches a power line fault detector that allows mechanical adjustment of the airgap of a concentrator for detecting various levels of current. This sensor may only be used for the detection of fault currents. Temperature and vibrations can induce errors in the readings of this form of detector.
Attarian et al., U.S. Pat. No. 6,844,799, teaches a Hall effect current sensor that utilizes mixed magnetic materials to optimize the dynamic range of the current sensor in a circuit breaker. The device requires fixed dimensions which cannot be adapted to some airgaps, and is therefore constrained with regard to magnetic strips that may be used.
Bosselmann et al., U.S. Pat. No. 5,963,026, discloses two Faraday elements or crystals for two different measurement ranges in order to achieve a higher dynamic range. This adds to the complexity and the cost.
Bluzer, U.S. Pat. No. 4,590,505, discloses a three dimensional optical receiver having programmable gain control. The gain is optimized in a logarithmic way which is not suitable to optical current and voltage sensors, which must be linear in order to maintain the accuracy of the harmonics.
The prior art teaches various devices and methods for measuring the current and the voltage in real time in a current carrying cable using optical sensors. However, the prior art does not teach a low cost and simple sensor design for accurate measurements at large dynamic range, sensitivity and bandwidth, that is capable of being installed on the cable without disturbing the function of the cable. The present invention fulfills these needs and provides further related advantages as described in the following summary.